A digital image used to process an image as digital data can be easily copied by a computer or the like and transmitted via a communication line without degrading the image quality, compared to a conventional analog image. This feature, however, makes it easy to illicitly copy and redistribute a digital image having a copyright or the like.
To prevent this, there is known a digital watermark method. Digital watermarks are roughly classified into an invisible digital watermark obtained by invisibly embedding watermark information such as copyright information or user information, and a visible digital watermark obtained by visibly forming in an image a watermark image such as the logotype of a company having a copyright.
As for the invisible digital watermark, embedded watermark information cannot be recognized or is hardly recognized in an embedded image at a glance. Watermark information is rarely deleted, but is illicitly copied and distributed more frequently than visible watermark information. Even if a digital image is illicitly copied or distributed, watermark information remains in the digital image. An illicit user can be specified by a user ID or the like embedded as the watermark information.
As for the visible digital watermark, watermark information is visibly written in a digital image. It is difficult to directly utilize the digital image, suppressing illicit copying and illicit distribution. As a conventional visible digital watermark embedding method, the pixel value of an image representing copyright information such as the logotype of a copyright holder is replaced with the pixel value of an original image, embedding copyright information in the original image. The drawback of this method is that the original image cannot be reconstructed without difference information because the pixel value of the original image is lost.
Japanese Patent Laid-Open No. 2000-184173 proposes a method of embedding visible watermark image shape information in an image by arithmetic processing (encryption) between all pixels or some bits and an embedding serial sequence at a position where a visible digital watermark is to be embedded. This method can implement a completely reversible visible digital watermark.
Japanese Patent Laid-Open No. 8-256321 proposes a method of extracting part of the bit string of an image which is compression-coded by JPEG or MPEG compression coding, directly converting the extracted bit string by an independently defined conversion method without referring to a part other than the extracted part, and decoding the image. This technique is called “semi-disclosure”, and can provide image information to the user while controlling the image quality of the disclosed image information. This method can implement a reversible visible digital watermark by designing a region where a bit string is to be converted.
It is desirable to remove such visible digital watermark by only a specific user having key information and hardly remove it by other users.
However, a visible digital watermark can be removed by the following attack.
Generally in a natural image, neighboring pixel values have a high correlation. Pixel values around a region where a visible digital watermark is embedded are analyzed. The pixel value of the region where the visible digital watermark is embedded is replaced with a value calculated on the basis of neighboring pixel values, thereby removing part of the visible digital watermark. Especially at a flat portion which hardly changes, an embedded visible digital watermark can be readily removed by this method. A conventional visible digital watermark has no proper measure against this attack.